1. Field of the Invention
The present invention relates to an image processing apparatus such as an apparatus provided with scanner, printer, or the like, and more particularly to an improved coding process.
2. Description of Related Art
In composite apparatuses provided with a scanner, printer, or the like, in order to reduce operation load in transmission/reception or storage of image data, image data is compressed by various coding processes to reduce the data amount. Of these coding processes, JPEG (Joint Picture Experts Group) coding is widely used as an international standard for image compression technology. However, when an image is processed using JPEG, only rough quantization is provided for high frequency components. Therefore, although significant image deterioration is not observed in a natural picture such as a photograph, images having many high frequency components, such as text and line art drawings, suffer from image deterioration called mosquito noise at boundary portions of differently colored regions.
On the other hand, because image data input to various image processing apparatuses including an apparatus provided with a scanner, printer, or the like, contains both photograph portions and text portions, uniform compression for such image data using JPEG results in undesirable partial image deterioration. It has been therefore proposed to separate input image data into a photograph portion and a text portion, and each portion is subjected to appropriate compression. For example, in MRC (ITU-TT.44 Mixed Raster Content) which is an international standard, image data is divided into a foreground image and a background image, which are switched using mask image data for compression by coding, and decoding.
More specifically, portions of image data suitable for JPEG, such as a natural picture, form a background image whereas other image data portions form a foreground image, and mask image data indicating the location of the foreground portion is generated. The background image is then compressed by JPEG coding and the foreground image is compressed by another coding method (such as predictive coding). In this manner, reduction in data amount and prevention of image quality deterioration is achieved.
The conventional approaches such as MRC, however, require image data separation as described above, and therefore require more than one coding device, which complicates the system configuration and increases operation load. There is therefore a demand for an image processing apparatus capable of reducing load in a coding process with a simple configuration.